1. Field of the Invention
The present invention is directed generally to Quantum Well Infrared photodetector Focal Plane Arrays (QWIP FPA""s) and, more particularly, to increasing the dynamic range of QWIP FPA""s.
2. State of the Art
Quantum Well Infrared Photodetector Focal Plane Arrays (QWIP FPA""s) are conventionally used for infrared detection and imaging. Typical applications of QWIP FPA""s include fiber optics communications systems, temperature sensing, night vision, eye-safe range finding, and process control. As is known in the art, QWIP FPA""s are composed of arrays of detector structures, wherein each detector structure produces a signal that is transmitted through a conductor bump to an external Read Out Integrated Circuit (ROIC) unit cell. The outputs of the plurality of ROIC unit cells associated with each detector in the array produce an integrated representation of the signal from the detector. To produce this output signal, a fixed bias is applied to the detector and the detector photocurrent resulting from the bias and the incident radiation is integrated. This integration function is performed by an integration charge well (integration capacitor) that is disposed within each individual ROIC unit cell. The combined integrated outputs of the plurality of ROIC unit cells in the array produce an image corresponding to the received infrared radiation.
As shown in FIG. 1, a conventional ROIC 100 maintains a constant bias across the QWIP photodetector 105 through the use of a direct injection transistor 110. Application of an operating bias Vbias 115 at the gate of the direct injection transistor 110 sets the maximum saturation current of the transistor and, in conjunction with the Detector Common bias voltage VDETCOM 120, determines the voltage across the QWIP. The photocurrent from the QWIP is integrated by the integrating capacitor (charge well) 125 which is connected to the output of the direct injection transistor 110. The integrating capacitor 125, in conjunction with the reset switch 130, performs a xe2x80x9cdump-ramp-samplexe2x80x9d (DRS) process to integrate the photocurrent i from the QWIP. One cycle of a DRS process typically involves first closing the Switch 130 to xe2x80x9cdumpxe2x80x9d any charge stored in the integrating capacitor 125 and then opening the switch 130 to allow the flow of charge from the QWIP to accumulate in the charge well 125 over an integration period xcfx84. The voltage on the charge well 125 xe2x80x9crampsxe2x80x9d during the charge accumulation period. When the multiplexer 135 is xe2x80x9cclosed,xe2x80x9d the voltage on the charge well 125 is xe2x80x9csampledxe2x80x9d by the subsequent read-out circuitry (e.g., amplifier, A/D converter).
QWIP photo-current, using a narrow-band flux approximation, is represented by the following:                               xe2x80x83                ⁢                              i            ⁡                          (              T              )                                =                                    τ              o                        ⁢            Ω            ⁢                          xe2x80x83                        ⁢                          A              d                        ⁢            η            ⁢                          xe2x80x83                        ⁢            gq            ⁢                          xe2x80x83                        ⁢                          Φⅇ                              (                                                      -                    Tp                                    T                                )                                      ⁢                          xe2x80x83                        ⁢            amperes                                              Eqn.  (1)            
where:
xcfx84o is the optical transmission efficiency,
xcexa9 is the optical solid viewing angle,
Ad is the pitch area of the detector in cm2,
xcex7 is the detector quantum efficiency,
g is the photoconductive gain,
q is the electron charge (1.6*10xe2x88x9219 coulombs),
"PHgr"p is the peak flux in photons/second/cm2/steradian,
Tp is the peak temperature in Kelvin, and
i(T) is the photocurrent in amperes.
As can be seen from Eqn. (1) above, the greater the peak flux ("PHgr"p) and the temperature (T) of the infrared source, the greater the flow of charge per unit time (i(T)=dq(T)/dt). Therefore, for any given level of peak flux ("PHgr"p), the temperature (T) of the infrared source will determine the rate of charge per unit time. The charge will thus accumulate in the charge well 125 faster at high temperatures than at low temperatures. The length of the integration period xcfx84 will further determine how much charge will accumulate in the charge well 125 for any level of photo-current from the QWIP. Infrared sources at low temperatures will cause a low rate of charge accumulation in the charge well 125. Therefore, only a small amount of charge will accumulate in the charge well over a short integration period. This can be problematic since the voltage on the charge well associated with the small amount of accumulated charge may not be sufficient to register in the noise floor of the A/D converter. The length of the integration period therefore effectively determines the lower dynamic range of the ROIC.
To overcome this problem, the integration period xcfx84 can be increased to permit a larger charge accumulation in the charge well as a result of the low temperature of the infrared source. The larger charge accumulation in the charge well will therefore advantageously raise the voltage across the charge well to a high enough level to register above the noise floor of the A/D converter. Correspondingly, however, increasing the integration period will permit high rates of charge, induced by high temperature sources, to accumulate quickly in the charge well. Accumulation of large amounts of charge in the charge well will likely cause a maximum saturation voltage to be reached across the charge well. Long integration times will therefore limit the ability of the ROIC to resolve high temperature sources without saturation, and thus will effectively limit the upper dynamic range of the FPA.
The conventional read circuitry shown in FIG. 1 therefore is deficient when the QWIP Focal Plane Array is used for imaging infrared objects that have greatly differing temperatures. The limitations on the dynamic range induced by a given integration period limits the ability of each detector to detect cold or hot objects with equal resolution. Thus, if the integration period is set to a length to adequately detect a cold object with sufficient resolution then the detector current output will likely saturate before detecting a hot object. In contrast, if the integration period is set to detect a hot object with sufficient resolution then the low current levels output from the detector will likely fall below the noise floor and thus not be sufficiently resolved by the analog-to digital converter. The conventional read out circuitry shown in FIG. 1 is therefore unable to maintain a sufficient dynamic range to adequately resolve the infrared radiation from both hot and cold objects.
Thus, it would be advantageous to construct a read out circuit that can resolve infrared radiation, from both hot and cold infrared sources, that is incident upon a quantum well photodetector.
An infrared photodetector focal plane array of exemplary embodiments of the invention includes large dynamic range Read Out Integrated Circuits. Increased dynamic ranges are achieved in each ROIC of the array using switched capacitor filter arrangements that include a single xe2x80x9ccupxe2x80x9d capacitor and at least two xe2x80x9cbucketxe2x80x9d capacitors. Dynamic range is improved by controlling the ratio of the current to voltage transfer resistance gains between the xe2x80x9ccupxe2x80x9d capacitor and each xe2x80x9cbucketxe2x80x9d capacitor of the filter. With an increased dynamic range, the switched capacitor filter arrangement allows the focal plane array to adequately resolve the infrared radiation received from both hot and cold objects.
One exemplary embodiment of the invention is directed to a method of sampling moving charges from a quantum well photodetector comprising the steps of: accumulating a first quantity of charges from said photodetector in a first charge storage device; supplying said first quantity of charges to a second storage device; accumulating a second quantity of charges from said photodetector in said first charge storage device; and supplying said second quantity of charges to a third charge storage device.
An additional exemplary embodiment of the invention is directed to a switched filter comprising: a first charge storage device for storing moving charges received from a quantum well photodetector; and switching means for selectively supplying a first quantity of charges from said first charge storage device to a second charge storage device and a second quantity of charges from said first charge storage device to a third charge storage device.
A further exemplary embodiment of the invention is directed to a focal plane array comprising: a plurality of quantum well photodetectors; a plurality of switched filters, each switched filter associated with a photodetector of said plurality of photodetectors, wherein each switched filter comprises: a first charge storage device for storing moving charges received from an associated photodetector; and switching means for selectively supplying a first quantity of charges from said first charge storage device to a second charge storage device and a second quantity of charges from said first charge storage device to a third charge storage device.